It has long been recognized that the lighter weight and better heat transfer properties make aluminum alloys a logical choice as a material for internal combustion engine blocks. However, most aluminum alloys lack wear resistance and it has been customary in the past to chromium-plate the cylinder bores in the engine block, or alternately, to apply cast iron liners to the bores. It is difficult to uniformly plate the cylinder bores, and as a result, plating is an expensive operation, and in the case of chromium plating, not environmentally friendly. The use of cast iron liners increases weight of the engine.
It is also recognized that there is a difference in the need for wear resistance between a four-cycle engine and a two-cycle engine. It has been found that there is a wear step in a four cycle cylinder bore area which is not seen in the two-cycle engine, and this wear step occurs where the piston and ring assembly changes direction from moving upward in the bore to downward in the bore. This fundamental difference occurs because the two cycle engine uses a charge of fuel and oil and thus lubricates the ring reversal area. Because of the less demanding wear requirements of a two-cycle engine, the bores of the two-cycle engine are frequently not honed and etched.
Hypereutectic aluminum-silicon alloys containing 17% to 19% by-weight of silicon possess good wear resistance achieved by the precipitated silicon crystals, which constitute the primary phase. Because of the wear resistance, attempts have been made to utilize hypereutectic aluminum silicon alloys as casting alloys for engine blocks to eliminate the need of plated or lined cylinder bores. However, the typical aluminum silicon alloy contains a substantial concentration of copper and when these alloys are used in humid or salt water environments, corrosion of the alloy can occur, with the result that alloys of this type are not acceptable as engine blocks for marine engines.
U.S. Pat. No. 4,603,665, describes a hypereutectic aluminum-silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of that patent is composed by weight of 16% to 19% silicon, 0.4% to 0.7% magnesium, less than 0.37% copper and the balance aluminum. This alloy has a narrow solidification range providing the alloy with excellent castability, and as the copper content is maintained at a minimum, the alloy has improved resistance to salt water corrosion.
U.S. Pat. No. 4,969,428 is directed to a hypereutectic aluminum-silicon alloy containing in excess of 20% by weight of silicon and having an improved distribution of primary silicon in the microstructure. The alloy of this patent contains from 20% o 30% by weight of silicon, 0.5% to 1.3% magnesium, up to 1.4% iron, up to 0.3% manganese, less than 0.35% copper and the balance aluminum. Due to the high silicon content in the alloy of U.S. Pat. No. 4,969,428, along with the uniform distribution of the primary silicon in the microstructure, improved wear resistance is achieved, making the alloy particularly suitable for use as an engine block for a marine engine.